"c000021a - XP BSOD on power down"

I am working on a friend's laptop, she has XP home edition. When it shuts down it has been restarting. I turned off automatic restart after system errors and finally got a BSOD, c000021a with status code 0x00000000 {0x00000000 0x00000000}. I've looked at MS's support site (http://support.microsoft.com/default.aspx?scid=kb;en-us;156669) and Dr. Watson hasn't found a problem, basically nothing on that site resolves this problem. I can't see how Winlogon can be a problem after Windows has seemed to shutdown, the BSOD comes after XP says "goodbye". I need to run some more scans, but she doesn't appear to have a virus/adware/spyware, she has AVG 7.5. I added Spybot but haven't run a scan yet. I'm currently running dskchk and will a memory test shortly. Windows runs fine until shutdown, I can't figure out what's causing it. None of event logs show a trace of an improper shutdown, BSOD, or other error. There is also no dump file. Dr. Watson can't find an error either, I think the error is after the kernal stops (could be wrong...). I'm at a loss for information right now. Please help!